Certain position-measuring devices or position-measuring instruments for linear or angle measurements based on scales having high-precision periodic graduation tracks are conventional. They are used in many types of machines to position axes with the aid of a drive. Examples for such machines are machine tools having three or more movable axes for positioning a tool and a workpiece relative to each other in accordance with an NC program, as well as pick-and-place machines or devices for processing wafers in electronics manufacturing.
In general, there is a trend toward increasingly higher demands on the accuracy of such machines, and therefore the demands on the position-measuring devices are tending to rise. Position measurements are already being performed in the nanometer range using scale-based position-measuring instruments rather than interferometers, which have traditionally been used in such range.
The scales of such position-measuring instruments usually have a periodic graduation track which is scanned by a detector of the position-measuring instrument. In so doing, periodic electrical signals, out of phase relative to each other, are generated, from which a shift is able to be ascertained, e.g., in a conventional manner, according to amount and direction.
Since, in addition to the shift in a measuring direction, the absolute position of the axis to be measured is often needed as well (e.g., for the commutation of a motor), absolute position-measuring instruments which are able to output a valid position immediately after being switched on are gaining increasing acceptance compared to incremental position-measuring instruments, which must first be brought into a reference position.
In addition to a very fine periodic incremental track, absolute position-measuring instruments include further tracks which permit the determination of an absolute position.
European Published Patent Application No. 1 400 778 describes that it is desirable to integrate reference markings at regular intervals into the incremental track having a small period. A second incremental track having a somewhat larger period is superimposed on the incremental track having a small period. In this manner, the assignment of an absolute track (the code connection) extending parallel to the incremental track is simplified considerably, and the susceptibility of the position-measuring instrument to moiré errors upon attachment is reduced.
According to European Published Patent Application No. 1 400 778, it is proposed that, in an incremental graduation structure made up of alternating light and dark regions, to replace one light (or dark) region by one dark (or light) region at regular intervals, e.g., by darkening an actually transparent region of a transmitted-light scale. This interruption of the periodicity of the incremental track is performed in each eighth period, and is therefore also referred to as a 1-out-of-8 gap.
Such a gap has an effect on the incremental signal similar to a periodic soiling of the scale. During the scanning of such a scale by a patterned photodetector having numerous sensor fields, which detects many periods of the scale simultaneously, such a disturbance of the periodicity at regular intervals does not impair the measurement so long as the scale and detector are optimally aligned relative to each other. However, if, for example, the illuminating optics do not supply an ideally parallel ray trajectory, or the photodetector is shifted radially in the case of circle graduations, then the image of the incremental track on the photodetector does not have the same graduation period as the photodetector. The image is either too large or too small, and a phase shift comes about from the middle of the sensor up to the two ends of the sensor field in the measuring direction. This phase shift is offset so long as, for a respective sensor on the one half of the scanning field, a sensor disposed in mirror symmetry with respect to the sensor middle can always be found on the other half. For example, these two sensors exhibit the same amount of phase shift, but an opposite operational sign. However, this is no longer the case when a soiling or simply a 1-out-of-8 gap extends through the sensor field. Individual sensors are covered, and the phase-angle error of the sensor thus not compensated for becomes effective. When forming a position value from the electrical signals of the photodetector, deviations thus result between the measured and the actual position value, and thus measurement inaccuracies arise.